The methodology of preparing polymer precursor powders comprising polyimide has been described in U.S. Patent Publication No. 2006/0033225, U.S. Patent Publication No. 2006/0033226, and PCT Patent Publication WO 2006/023419.
U.S. Pat. Nos. 5,172,307, 5,369,546, 5,776,633, and 5,973,912 describe AC/C composites produced by applying phenolic resin as a binder to activated carbon powder followed by curing the resin and pyrolysis. The carbon phase derived from phenolic resin is a dense carbon without sufficient pore surface area for many applications. Thus, the AC/C composites prepared using these methods had pores blocked by the binder and had significantly reduced surface area compared to the conventional activated carbon powders used as electrode materials.
Porous carbon phases, which bind to other materials, possess interpenetrating pore structure, high density, and high surface area, are highly desirable for making porous carbon foam composites. The previous art as described in U.S. Patent Publication 2006/0033226 has provided a group of polymer binders which, after pyrolysis, produce such porous carbon foam phases.
Polyimide precursors are prepared from monomers of aromatic dianhydrides, aromatic diamines, and optionally aromatic polyamines with amine functionality of two or greater. In a typical procedure, the monomers are dissolved in an organic solvent, such as dimethylacetamide, with stirring to form a viscous solution of poly (amic acids). The imidization is carried out to produce polyimide precipitates. The solvent is removed from the product by distillation. As an option, the polyimide precursor powder is thermally annealed at 300-500° C.
In the present invention, the polyimide precursor powders of U.S. Patent Publication No. 2006/0033225, U.S. Patent Publication No. 2006/0033226, and PCT Patent Application US2005/028890, are used in a process for forming activated carbon/carbon (AC/C) composites and porous carbon foam composites. The polyimide precursor powders are self binding and bind to other materials under a compression pressure to form a monolith. The carbon phase derived from the polymer precursors after pyrolysis possesses high surface area above 600 m2/gram and high density above 0.7 gram/cc. The present invention provides porous carbon foam composites, methods of producing, and applications, of porous carbon foam composites applying the polyimide precursors for the carbon phase and incorporating other active materials therein. Applications of porous carbon foam composites include, but are not limited to, use as electrodes or as a combined electrode and current collector for electrochemical capacitors, asymmetric capacitors, batteries, lithium ion batteries, fuel cells, sensors, water treatment facilities, for use as adsorption media for filters, and for use as catalyst support, among other uses.
The present invention provides monolithic porous carbon disks prepared by compressing the polymer precursor powder into a monolith at ambient temperature followed by pyrolysis under protection of an inert atmosphere. The porous activated carbon/carbon composites were prepared by mixing the polymer precursor and activated carbon powder and, as an option, carbon fiber into a homogeneous mixture and then compressing the mixture into a monolith followed by pyrolysis under protection of an inert atmosphere. The carbon phase derived from the polymer precursor possesses high surface area at nano- and micro-pore region and high density in this embodiment.
AC/C composites as described herein have application in electrochemical capacitors. An electrochemical capacitor comprises at least one electrochemical cell and at least two current collectors which are connected to at least a first and a second electrode, respectively. A porous separator is sandwiched between the two electrodes, and liquid electrolyte impregnates the pores of electrodes and separator. The present invention produces such electrochemical capacitors in which at least one of the electrode plates comprises AC/C composites. AC/C composites may be used in both the electrode and the current collector.
For electrochemical capacitors comprising two or more electrochemical cells which are connected in series, the adjoining cells are separated by a cell separator which is an electron conductor and ion insulator. The cell separator may be bound to each of the adjacent electrodes to form bipolar plates. Such bipolar plates may comprise AC/C composites as described herein as electrode plates bound by a cell separator.
When the AC/C composites of the present invention were applied as electrodes for electrochemical capacitors, the specific capacitance of the AC/C composite is found to be 2 to 4 times higher while the equivalent series resistance (ESR) is significantly lower compared to the conventional carbon electrodes.